The subject matter described herein relates generally to electric machines, and more specifically, to methods, systems, and apparatus for reducing cogging torque in an electric machine.
Electric machines, including electric generators and electric motors, are used in countless varieties and applications worldwide. Typically, the rotational force and torque generated within an electric motor is delivered to an application by a rotor shaft. The torque generated is a product of current applied to the motor and an electromagnetic field maintained in the motor. This delivered torque varies as a function of time and position of the rotor. Magnets on the rotor generate a rotor magnetic field and current on a stator winding generates a stator magnetic field. When the rotor generated magnetic field approaches the stator generated magnetic field, the torque is positive, and when the rotor magnetic field leaves the stator magnetic field the torque is negative. The torque produced is therefore non-uniform and known to those in the art as torque ripple. A second component of non-uniform torque is known as cogging torque. Cogging torque is present because the rotor magnets prefer to line up with the stator teeth. In some applications, the ripple and/or cogging torque produces objectionable vibration at the motor shaft resulting in end product noise. Furthermore, ripple and/or cogging torque may produce undesirable stator torsional and/or radial forces
One example of such an application occurs when a motor drives an end product, for example, a fan. Cogging torque produces vibrations which are transmitted to machine components such as the motor and fan mounting. These vibrations produce undesirable noise as the cogging frequencies couple with ‘application’ resonances. In addition to acoustic noise, continued exposure over time to such vibrations may loosen motor and fan assemblies, and ultimately may cause a motor failure. Isolation and damping systems, for example, an isolated rotor, may be employed to minimize the effects of the vibrational energy induced into the motor and fan system.
Furthermore, in applications that include a rotor that includes permanent magnets, such as a brushless direct current (BLDC) motor or a brushless alternating current (BLAC) motor, a resultant noise due to inherent cogging torque is caused by rotor permanent magnets moving past stator teeth. Cogging torque may be reduced by including a skewed magnetic field. However, it is currently difficult to apply a skewed magnetic field in a motor that includes an interior permanent magnet rotor. Adding sub-slots to the stator teeth will reduce the composite peak-to-peak cogging torque, but adds higher frequency components that can excite system resonances in some applications. Cogging torque at frequencies other than the fundamental frequency may also cause motor vibration and generate noise in an end product. Furthermore, cogging torque may be reduced by using a resilient rotor construction. However, a resilient rotor significantly increases a cost of the motor, while adding a potential failure mechanism to the motor. In addition, there may also be undesirable interactions between the magnetic field generated by the stator windings and the back EMF of the PM motor that may create torque pulsations (i.e. torque ripple) rich with harmonic content.
Moreover, efficiency of BLDC and BLAC motors with permanent magnets embedded in the rotor (e.g., an interior permanent magnet rotor) is typically limited due to leakage flux of individual permanent magnets through the rotor core.